At present, post-processing of sheets that requires a relatively long processing time typically involves stacking multiple sheets one on top of another before the sheets are conveyed to a post-processing means so that multiple sheets can be conveyed at a time, thus minimizing productivity loss.
For example, a preliminary stacking channel in which multiple sheets are retained may be provided upstream from the post-processing means, such as an alignment tray, in a direction in which the sheets are transported (sheet conveyance direction). When the number of sheets stacked preliminary in the preliminary stacking channel reaches five, a subsequent sheet (i.e., sixth sheet) and the five sheets stacked preliminary are discharged to the alignment tray at a time. This conveyance method can save time compared with a case in which the sheets are conveyed singly, one at a time. Accordingly, productivity loss can be minimized, particularly in processing operations such as stapling at multiple positions, which requires a relatively long time.
The image forming apparatus may transmit sheet data and post-processing type to the sheet processing apparatus including the preliminary stacking channel, and time required for post-processing calculated based on the transmitted data is reported to the image forming apparatus. Then, an interval between two consecutive sheets discharged from the image forming apparatus to the sheet processing apparatus is adjusted according to the reported processing time. In this method, loss in productivity caused by post-processing of sheets can be reduced, and, if the processing type is changed, such changes can be reflected properly.
In configurations in which multiple sheets are retained in the preliminary stacking channel, it is preferable to variably control the length of the discharge intervals between sheets. Specifically, although a single sheet interval between two consecutive sheets is typically increased to secure post-processing time, when multiple sheets are stacked preliminary, discharge intervals of sheets discharged from the image forming apparatus for optimum productivity depends on the number of sheets stacked preliminary.
Accordingly, it is necessary to change the discharge intervals according to the number of sheets stacked preliminary. However, such control is complicated, increasing both the time necessary for development as well as the number of software bugs. Overcomplicated control of sheet conveyance can be avoided by increasing the length of the discharge intervals, but at the cost of a decrease in productivity.